1. Technical Field
The present invention relates to an exposure head adapted to image light from a light emitting element with an imaging optical system, an image forming apparatus using the exposure head, and a control method of the exposure head.
2. Related Art
As such an exposure head, there is described in JP-A-2008-36937 an exposure head having one imaging optical system disposed with respect to a plurality of light emitting elements. The imaging optical system images the light beams from the corresponding plurality of light emitting elements. Then, the imaged light beams expose the exposed surface.
Incidentally, it has been known in the past that the light emitting elements are deteriorated while repeating emission of light, and thus the light intensity is reduced. Further, if such reduction of light intensity occurs, the exposure head might fail to execute a preferable exposure operation. To cope with this point, there is proposed in JP-A-2004-82330 (Document 1) a light intensity control technology for realizing a preferable exposure operation irrespective of the deterioration of the light emitting elements. In this light intensity control technology, the light emitting elements are sequentially driven to emit light in a pre-shipment inspection of the exposure head, and the light intensity of the light thus emitted from the respective light emitting elements is detected by a light intensity sensor. Further, after the shipment, an inspection similar to the pre-shipment inspection is also executed at timing, for example, between an interval of exposure operations or upon powering on. Further, the degree of deterioration of the light emitting elements is obtained based on the light intensity detected in each of the inspections before and after the shipment. Specifically, a proportion (a “correction coefficient” of the Document 1) between the light intensities detected before and after shipment is obtained. By controlling the light intensity of the light emitting elements based on the proportion thus obtained, the light intensity of each of the light emitting elements is equalized irrespective of the deterioration, thereby making the preferable exposure operation possible.
However, the light intensity of the light emitting element also varies with temperature variation. Therefore, if the temperature of the light emitting element is different between the light intensity detection before shipment and the light intensity detection after shipment, the light intensity varies not only by the deterioration but also with temperature variation. As a result, the degree of deterioration might not be obtained accurately in some cases, because the degree of deterioration obtained from the light intensities detected before and after shipment is influenced by the temperature variation. Further, in such a case, there is a possibility that the preferable exposure operation is not executed because the light intensity variation due to the deterioration cannot properly be controlled.